The present invention relates to a method for producing a silver halide photographic emulsion.
The production of silver halide photographic emulsions is usually carried out by adding silver ions and halide ions into reaction vessels equipped with stirrers. Nucleation occurs by initial addition, and crystal growth is performed by subsequent addition. Stirring methods include various methods, for example, as described in JP-A-7-219092 (the term xe2x80x9cJP-Axe2x80x9d, as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-8-171156, JP-A-4-283741, JP-B-8-22739 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) and U.S. Pat. No. 3,782,954. However, when nucleation is conducted by such methods, nucleation and nuclear growth occur in parallel even by any of these stirring methods, because solutions circulate in reaction vessels. It is therefore difficult to form monodisperse nuclei.
In the field of silver halide photography, tabular silver halide grains having large light-receiving areas have been widely used as light-sensitive elements. For increasing the light-receiving efficiency, thin tabular silver halide grains are preferred. However, according to the methods as described above, the tabular silver halide grains in the course of growth pass through a high supersaturation region in the vicinity of an opening for adding silver ions or halide ions to cause the harmful effect that the tabular grains are increased in thickness.
For solving these problems, there is a method of providing an external mixer in addition to the reaction vessel, forming fine silver halide grains with the external mixer, and using them in the nucleation stage or the growth stage. According to this method, an aqueous solution of a silver salt, an aqueous solution of a halide and an aqueous solution of a dispersing medium are added to the external mixer to continuously form fine grains. The fine grains can be used for nucleation and/or growth. In such a method, it is desired that the mixer can completely mix the added solutions for as short a time as possible. It is unfavorable that a long time is taken for mixing, or that the added solutions circulate in the external mixer.
As the reaction vessels, various type ones can be used. For example, in U.S. Pat. No. 5,250,403 and JP-A-10-43570, mixing is performed with an agitating blade in a small-volume mixer. However, in such a method, the added solutions circulate in the mixer.
In JP-A-4-139440 and PCT International Publication JP-A-6-507255, mixing is performed without mechanical stirring, so that there is no circulation of the added solutions. In this method, however, the mixing power is insufficient because of absence of stirring.
For maintaining the sufficient mixing power without mechanical stirring, there is a method of turning the added solutions into jet streams, and conducting mixing by their kinetic energy. JP-A-8-334848 discloses a method for producing a silver halide photographic emulsion utilizing kinetic energy of such jet streams. However, the patent is directed to a method for producing a silver halide photographic emulsion by the single jet process, and utterly different from a method using the external mixer. Further, the kinetic energy used herein is insufficient for mixing the whole contents of the reaction vessel, so that mechanical stirring is used in combination.
It is therefore an object of the present invention to provide a method for producing a silver halide photographic emulsion, which can continuously form small-sized and monodisperse silver halide grains. Another object of the present invention is to obtain monodisperse silver halide grains by using the above silver halide photographic emulsion as nuclei. A further object of the present invention is to make it possible to form thin tabular silver halide grains by using the above silver halide photographic emulsion for crystal growth.
The objects of the present invention are attained by the following:
(1) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and mixing the two kinds of solutions with each other for a short time, thereby continuously forming silver halide grains;
(2) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and mixing the two kinds of solutions with each other without their circulation, thereby continuously forming silver halide grains;
(3) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and mixing the two kinds of solutions with each other without mechanical stirring, thereby continuously forming silver halide grains;
(4) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and passing the two kinds of solutions through a capillary parallel to the jet stream and having recesses on the inside thereof to mix them with each other for a short time, thereby continuously forming silver halide grains;
(5) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and passing the two kinds of solutions through a capillary parallel to the jet stream and having recesses on the inside thereof to mix them with each other without their circulation, thereby continuously forming silver halide grains; and
(6) A method for producing a silver halide photographic emulsion comprising turning at least one of an aqueous solution of a silver salt and an aqueous solution of a halide into a linear jet stream having a high flow rate, and passing the two kinds of solutions through a capillary parallel to the jet stream and having recesses on the inside thereof to mix them with each other without mechanical stirring, thereby continuously forming silver halide grains.
In the invention, an aqueous solution of silver nitrate is usually used as the aqueous solution of the silver salt. When the silver halide grains obtained in the method of the present invention are used as nuclei, the concentration of the aqueous solution is preferably 4 mol/liter or less, more preferably 1 mol/liter or less, and most preferably 0.2 mol/liter or less. When the grains are used for crystal growth, the use of an aqueous solution having a high concentration is preferred from the viewpoint of productivity. The concentration thereof is preferably from 0.5 mol/liter to 4 mol/liter, and more preferably 1.0 mol/liter or more. The temperature of the aqueous solution is preferably from 5xc2x0 C. to 75xc2x0 C.
The aqueous solutions of the halides usually used in the present invention include aqueous solutions of potassium bromide, sodium bromide, potassium chloride, sodium chloride, potassium iodide, sodium iodide and mixtures thereof. When the silver halide grains obtained in the method of the present invention are used as nuclei, the concentration of the aqueous solution is preferably 4 mol/liter or less, more preferably 1 mol/liter or less, and most preferably 0.2 mol/liter or less. When the grains are used for the crystal growth, the use of an aqueous solution having a high concentration is preferred from the view point of productivity. The concentration thereof is preferably from 0.5 mol/liter to 4 mol/liter, and more preferably 1.0 mol/liter or more. The temperature of the aqueous solution is preferably from 5xc2x0 C. to 75xc2x0 C.
It is preferred that at least one of the aqueous solution of the silver salt and the aqueous solution of the halide used in the present invention contains gelatin as a protective colloid. Gelatin has a significant effect on the frequency of the occurrence of twin crystals in the silver halide grains formed, so that the preferred concentration of an aqueous solution of gelatin varies depending on the purpose for use of the fine silver halide grains formed.
When the silver halide grains continuously formed are used as nuclei in preparing tabular silver halide grains, parallel double twin crystal nuclei are necessary. It is therefore necessary that the concentration of the aqueous solution of gelatin is adjusted so as to achieve the desired frequency of the occurrence of twin crystals. The gelatin concentration is preferably selected so as to give a gelatin amount of 0.03 g to 0.4 g, more preferably 0.3 g or less, per gram of silver, when the aqueous solution of the silver salt and the aqueous solution of the halide are mixed with each other.
The fine-grained silver halide emulsion obtained by the present invention can be used as nuclei in the crystal growth of the silver halide grains. When the grains are utilized for the crystal growth, it is preferred that the silver halide grains added are rapidly dissolved. Accordingly, less twin crystal nuclei are preferred, and it is preferred that the aqueous solution of gelatin has a higher concentration. The concentration of the aqueous solution of gelatin is adjusted to such a concentration that preferably 0.2 g to 1 g, more preferably 0.3 g or more, and most preferably 0.4 g or more, of gelatin is added per gram of silver nitrate added.
When the concentration of the aqueous solution of gelatin is increased, the viscosity of the aqueous solution of gelatin increases to make addition thereof difficult. Decreasing the molecular weight of gelatin by techniques such as enzymolysis can decrease the viscosity. The molecular weight of gelatin is preferably from 5,000 to 100,000, more preferably 50,000 or less, and most preferably 30,000 or less.
When utilized for the crystal growth, gelatin added together with the silver halide grains has an effect on the thickness of the tabular silver halide grains. The effect on the thickness can be variously changed by chemical modification. For obtaining thin tabular silver halide grains, oxidation treatment, succination treatment and trimellitation treatment can be preferably used.
The flow rate of the solution added as the jet stream is preferably 100 m/second or more, more preferably 250 m/second or more, and most preferably 500 m/second or more.
The diameter of the capillary in which the solutions are mixed with each other is preferably 20 times or less, more preferably 10 times or less, and most preferably 7 times or less, the diameter of an opening for adding the linear jet stream.
The length of the capillary in which the solutions are mixed with each other is preferably 10 times or more, more preferably 50 times or more, and most preferably 100 times or more, the diameter thereof.
The capillary has at least one recess on the inside. When the solution added flows through the capillary, the recess converts the flow to finer turbulence to cause more homogeneous mixing.
When mixing is conducted by the jet stream having a high flow rate, the temperature of the mixed solution is elevated. It is therefore preferred that the apparatus is equipped with a cooler.
The aqueous solution of the silver salt and the aqueous solution of the halide are preferably mixed with each other for a short time. The mixing time is preferably 0.5 second or less, more preferably 0.1 second or less and most preferably 0.05 second or less.
When the aqueous solution of the silver salt and the aqueous solution of the halide are mixed with each other, it is preferred that the circulation of the added solutions does not occur in the external mixer. In the case that the added solutions circulate in the external mixer, the growth of the resulting fine silver halide grains is unfavorably liable to occur.
It is preferred that the mixing of the aqueous solution of the silver salt and the aqueous solution of the halide is not accompanied by mechanical stirring. When the mixing is accompanied by the mechanical stirring, it is difficult to mix the solutions without the circulation. Further, when the mixing time is as short as 0.1 second or less, it is difficult to sufficiently mix the solutions by the mechanical stirring.
Mixing methods satisfying the requirements of the present invention include a method using a high pressure homogenizer (DeBEE 2000) manufactured by BEE INTERNATIONAL Co. Using the dual field process of the homogenizer, one of the aqueous solution of the silver salt and the aqueous solution of the halide is turned into a high speed jet stream, which can be mixed with the other solution. The application of high pressure to the aqueous solution to be turned into the jet stream gives high kinetic energy to the solution, which makes it possible to mix the two solutions with each other for an extremely short time. According to this method, the circulation that the solution added return to the vicinity of the addition opening again is not generated, and further, the mechanical stirring is not required because the added solution has sufficient kinetic energy.